i’d keep the modem and router separate just to keep the load of modem duties off of the switching duties of the router. Dell are fine. Don’t dismiss Netgear, though. I have been using Netgear routers, switches and access points for years. Currently have some 30 odd wired devices and about 8 WiFi devices supported by my old (3 years) router… it has been astonishingly reliable (I cannot actually remember when I last needed to reboot it… perhaps no more than once or twice in three years), and the only network faults I have experienced have been down to VM.

no open ports here! traffic is regularly checked using wire shark. I did an experiment with a undefended machine in our Cyber Security Centre at work, and from it going live it took 9 minutes for it to be port scanned by bots…

You could in any case consider to name the 2.4 and 5 MHz bands differently.
So you have 2 wifi networks, one of which you can dedicate to the heavy traffic application.
If you add a hub or router additionally, you have 4 wifi networks.
Each will handle up to 4 devices, I guess that would be OK.

I was running an ASA5506 on the edge but have now switched to a Sophos UTM and it’s doing a good job, currently running a vmware lab with 7x hosts and around 200x VM’s at home so the network is constantly under attack, the sneaky bastards are trying more extensive portscans to find a hole rather than the general nmap ones.